Low-energy spin excitations in Ni over-doped NaFe0.7Ni0.3As

The discovery of iron-based superconductors gives a new horizon to study unconventional superconductivity. Typically iron-based superconductors can be created by doping (i.e. replace Fe ions with) Co or Ni in some parent compounds such as NaFeAs. It is believed the magnetism in these materials is strongly coupled to superconductivity. One way to probe the magnetism in these materials is to observe the spin gap by inelastic neutron scattering. Our previous experiment shows no gap for the parent compound and a spin gap at 11% Co doping (11% electron doping), however, no gap is observed at 60% electron doping, contrary to the previous characterizations. This experiment is to take a closer look to confirm the true size of the spin gap (if it exists) at 60% election doping.